Early in
2004 I received a letter from Dean, VK4ALN, generously offering me an
HRO 50T1 from the early 1950's. Dean said that it had been stored in a
cupboard for many years and while it was not in good condition, I was
welcome to it if I intended to restore it. While it was certainly very
rusty, it was decided that the best approach was not to strip it right
down for a replate as had been done for the AR7, but that as much rust
would be scraped off as possible and clear varnish would be applied to
the chassis and of course fresh paint on the cabinet after a major sand
down. It had a burnt out mains transformer and an old TV power
transformer was located that had similar voltage ratings to the old one,
but much higher current capabilities. It was installed. The unit had no
audio output transformer, as it had burnt out too. The output was a pair
of 6V6's in push pull, and so a replacement audio transformer was
fitted. Next, all the resistors and most of the capacitors were replaced
with new ones.

Circuitry
wise, the HRO 50T1 is quite curious. It is a very standard circuit for a
15 valve, single conversion receiver, but the front end consists of two
stages with 6BA6's, a mixer using 6BE6 then a 6C4 as the HF oscillator.
That is the end of the miniature tubes. From there it uses a 6K7 and two
6SG7s in the IF and a 6H6 detector. The BFO is a 6J7 although I found
the 6K7 worked better. There is a 6SN7 as S metre driver and audio phase
splitter, a 6SJ7 as an AF amplifier and then the 6V6's as push pull
output. A 6H6 performs the task of adjustable noise limiter. One can
only assume that National had a stock of the older octal tubes that they
wanted to use up, but they used the four 7 pin miniature tubes in the
front end for their improved performance. I have now removed the first
6H6 (detector stage) and in place have wired in a 6SA7 as a product
detector to improve SSB performance. There was only a slight reshuffle
of wires on the mode switch, and no changes to the front panel. A 7 pin
tube socket has been inserted under the chassis near the old 6H6 socket
and a 6AL5 has been used in place of the 6H6. I was using a pair of
germanium diodes in place of the 6H6 at one stage and they worked fine,
but why use solid state when one can use a tube?

IMPORTANT SAFETY
NOTE: If anybody ever restores a receiver of this type, please note that
an asbestos gasket is fitted between the RF chassis and the Power
Supply / audio output chassis probably to reduce heat transfer. This
asbestos is extremely dangerous and the fibres must not be inhaled. This
was one reason why I did not pull the receiver apart for replating. I
sealed the asbestos with clear varnish, brushing plenty in and allowing
it to set. I have recently found more asbestos in an old broadcast
receiver I was restoring. A sheet measuring about 3" x 10" had been used
to stop tube heat from heating up the plastic on the top of the cabinet.
It was dealt with the same way with varnish.

Original state

After restoration

The paint on
the coil box is still original at this stage. Comparing the paint colour
on the coil box to that on the cabinet will give a good idea of the
degree of colour match achieved. The original paint was slightly
metallic and the fresh paint is more so.

I will take
some inside photos of the old receiver next time it is open and add them
to this page